This application claims the benefit of U.S. Provisional Application No. 60/817,428, entitled “Long Coil Pair for Tracking the Drilling of a Borehole”, filed Jun. 30, 2006, the disclosure of which is hereby incorporated herein by reference.
The present invention relates, in general, to a method and apparatus for measuring the relative locations of boreholes and for drilling boreholes that are accurately placed relative to each other. More particularly, the invention relates to a drilling guidance tool that is deployed in an existing reference borehole, the tool incorporating a coil assembly having an elongated core carrying a pair of elongated crossed coils that are energized with alternating current so as to produce a rotating, elliptically polarized magnetic field. An instrument containing magnetic field sensors is deployed in a second borehole that is being drilled to measure the rotating magnetic field and to track and to guide the drilling.
The technology for accurately tracking and drilling boreholes in a known location in the Earth using electromagnetic techniques has been well developed over the years. Also, methods exist for accurately tracking and drilling boreholes that are to be positioned relative to existing boreholes in areas where direct measurement from the surface is not possible. One example of such methods uses a long solenoid coil deployed in an existing borehole to generate a known magnetic field (either DC or AC). The magnetic field generated by this coil is measured in the second borehole, and these measurements are used to calculate the position of the second borehole relative to the first. Another method uses a long thin coil of wire wrapped lengthwise around a section of plastic pipe in the existing borehole. Measurements of the orientation of this coil along with measurements of the magnetic field produced by the coil in the borehole being drilled are used to compute the position of the second hole. However, problems exist with these and other current methods. For example, when using a solenoid coil to produce the magnetic field that is to be measured, the solenoid must be continuously moved along the first borehole as the second is being drilled; generally it must be moved for each new position measurement. Methods using long thin axial coils allow several distance measurements to be taken before the coil must be moved, but it is necessary to know the rotational orientation of the long thin coil in order to compute the second borehole location. Measuring or setting this rotational orientation is often difficult, in practice, and this adds to the complexity and cost of the drill guidance system.
Numerous patents exist that disclose the use of electromagnetic sources in a reference borehole in the Earth to track and to guide the drilling of a second borehole. For example, U.S. Pat. No. 3,853,185 to Dahl discloses the use in a reference borehole of a loop antenna excited by radio frequency (RF) alternating current and the use of another antenna on drilling apparatus in a second borehole to receive the generated signal. Both direction and distance to the reference well from the borehole being drilled are determined from the received signals. In addition, U.S. Pat. No. 6,927,741, to Brune et.al., discloses the use of a transmitting loop antenna, a mechanism for measuring the roll angle of the transmitting loop, and magnetic field receivers to measure the generated electromagnetic field components to determine the relative orientation of, and the distance between, the transmitting loop and the receivers. In still another example, U.S. Pat. No. 5,923,170 to Kuckes discloses the use of an arbitrary wire loop of known configuration, including a loop with wire segments in a borehole. U.S. Pat. No. 4,875,014 to Roberts et al discloses another method of using loops on the ground for determining drilling location, as does U.S. Pat. No. 3,589,454 to Coyne. U.S. Pat. No. 5,589,775 to Kuckes discloses the utility of the ellipticity of the rotating electromagnetic field produced by a rotating magnet to track the drilling of a borehole.
One prior approach to measuring the rotational orientation of a long coil has been to deploy a tilt-sensing instrument with the coil. Such a tilt sensor measures the angle between the plane of the coil and the direction of gravity. A problem with this approach is that it does not work at all for vertical holes. In such cases, the rotational orientation of the coil must be determined; for example by connecting a rigid structure (such as a pipe) to the coil and measuring the rotational position of the pipe at the surface of the Earth. This can be problematic in practice, however, since the rigid structure may twist as it goes down into the borehole, making it difficult to accurately measure the rotation angle of the coil with respect to some fixed point on the surface.
Accordingly, there is a need to be able to measure the location of a second borehole relative to an existing first borehole which avoids the foregoing and other problems encountered in the use of current drill guidance equipment.